Similar to the original post, another thing I never use is overloaded operators. (In any language.) More generally I don't like anything which changes the syntax, or "basic meaning" of a language. (I'm not sure which term to use here.) Tied variables fall into this category, as do lvalues.
Basically I want a Perl hash to act like a Perl hash; if it's not exactly like a Perl hash, then it should be a SomeOtherHash object. I want an equal sign to mean the same thing anywhere I see it; if I need something else, use an is_equal_in_some_way() sub. I want a sub to be a sub, not something I can assign to.
It's not that there's anything inherently wrong with those things, it's that they aren't Perl, or that they change Perl in a fundamental way. It's easier to read code when we're all speaking the same language.
Better to combine existing words into meaningful phrases than to make up new words. Better to make up new words than to change the meaning of words I already understand to mean something else. "From now on, 'green' means 'tired'. I sure am green today!" That's very confusing to me.